The present invention relates to the determination of the location of an object through the combined use of a route database and satellite measurement. More specifically, the invention relates to location determination of a train through the use of a track database and data from just two global positioning satellites.
The ability to locate trains across a wide area is becoming of increasing importance. Resource limitations inherent in any track system such as the number and availability of sidings, loaders, unloaders, switches, and personnel make the intelligent use of such resources of utmost importance. These resources are best used through the efficient transit of trains to and from these resources. Efficient transit can only be accomplished if the locations of the various trains in the system are known with accuracy.
The Global Positioning System (GPS) and other satellite based location determining systems have been available and in use for a number of years (the term GPS is used hereafter to denote any positioning system which uses satellites and has capabilities similar to those of the GPS system.) Use of GPS systems with a wide variety of vehicles, including trains, is known to the field. Also known to the field are the inherent limitations of GPS use.
An accurate GPS location determination requires a GPS receiver to receive signals from four different GPS satellites. A train or any other vehicle can easily receive signals from the four required satellites if the vehicle is located in an open area, free of signal obstructions. For this reason, ships at sea and airplanes in flight are well positioned to make full use of GPS to accurately determine their location. A train located in an open area can similarly receive signals from the required four satellites. However, trains are not always so conveniently located.
The very nature of train travel is such that trains will be found in locations where they cannot easily receive from four satellites. Trains travel next to tall, signal obstructing structures, both natural and man-made. Trains travel through canyons and other areas which interfere with signal reception. As such, trains are often in the situation, unique from some other forms of mass and freight transit, in which they can receive signals from fewer than the required four satellites, and frequently can receive signals from only two satellites.
Obviously, there are other methods for determining the location of a vehicle. Particularly with respect to rail-based transportation, it is possible for a vehicle to have access to a database of information pertaining to rail routes whose locations are fixed and known. Such a database may be used to provide a way of converting elapsed distance from a known point along a known route into a location in two or three dimensional coordinates.
Such a system is well suited to rail vehicles by virtue of the fact that these vehicles cannot stray from their fixed and known tracks. The advantages of such a system are limited by its logistics, however. In order to know the distance traveled from a fixed point, an odometer type of measurement must be taken. Such a measurement is generally taken by counting wheel rotations, which is fraught with inaccuracies: wheels slip on rails, potentially both during acceleration and braking; wheel diameter changes over time as wheels wear down and develop flat spots; any wheel rotation measurement and calculation method is inherently at least partly mechanical, thus subject to mechanical problems; all such measurements are based on correctly resetting a counter at a designated zero point from which such measurements are taken, which might not be easily performed; and independent of the ability to measure distance travelled, the entire system is subject to the accuracy of the initial database upon which the final location determination is based.
It is desirable to combine the best features of satellite based and elapsed distance based location determination methods. Such a system could approximate a rail vehicle's location based on a track database to within some range of error. This estimate could be used as the basis for a satellite based measurement which takes into account not only the estimated location of the rail vehicle, but also the relative location of nearby geosynchronous satellites. Such a system need not have access to the full four satellites normally required.
Accordingly, it is an object of the present invention to provide a novel method of determining the location of a vehicle whose movement is constrained to a route whose location is fixed and known which obviates the problems of the prior art.
It is a further object of the present invention to provide a novel method of determining a vehicle's location along a fixed route by using a database of route locations in conjunction with positional information from just two satellites from a global positioning system.
It is another object of the present invention to provide a novel method of determining the location of a vehicle by estimating the vehicle's location along a route, determining angles from a vector tangent to the route to each of two satellites, determining range errors between an estimated range from the estimated location of the vehicle to each satellite and a measured range from the vehicle to each satellite, and using the determined angles and determined range errors to determine a distance error which is combined with the estimated location of the vehicle to determine its location.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.